A heat source unit of an air conditioner is usually equipped with a compressor, a fan, or another such electrical device that produces comparatively loud operating sounds, and such operating sounds are sometimes the cause of noise. In addition to a regular operating mode, the heat source unit disclosed in Japanese Laid-open Patent Application No. 7-103546 is provided with a reduced-noise operating mode in which a reduced-noise operation is achieved by imposing an appropriate restriction on the upper limit of the operating frequency of the compressor or the rotational speed of the fan.